1. Field of the Invention
The present invention relates to an information reading and recording apparatus for recording media, such as an optical disk.
2. Description of the Related Art
In the current information input/output apparatus using light, such as CD (compact disk) drive, light emitted from a laser beam source is converged as a micro spot constituting a recording pit on a track provided on a disk-like recording medium such as a CD, presence or absence of the pit is recorded as information, and the presence or absence of the pit on the track is detected by means of reflected light from the track thereby reading the information.
Recently, DVDs, which have a recording capacity about 7 times as large as that of CDs, are becoming remarkably popular along with the demand for an increased recording capacity. Increase in recording capacity means improvement of the recording density, which depends on the number of recording pits formed on a recording medium (hereinafter referred to as “disk”). In DVDs, one factor in increasing the recording density is to decrease the size of a recording pit, that is, decrease the diameter of a spot of laser beam radiated on the disk. The size of the micro spot to be radiated on the disk is proportional to the wavelength of the laser beam and is inversely proportional to the numerical aperture of an objective lens. Accordingly, for increasing the recording density, it is required to shorten the wavelength of the laser beam and to increase the numeral aperture of the objective lens.
However, DVDs are strongly required to be compatible with CDs from the viewpoint of backward compatibility of software. Originally, an optical head device was provided with one laser beam source with a wavelength of 635 to 650 nm and one objective lens having a numerical aperture of about 0.6 for the DVDs, and also with another laser beam source with a wavelength of 780 nm and another objective lens having a numerical aperture of about 0.45 for CDs, thereby ensuring the compatibility between the both disks.
However, when the numerical aperture of the objective lens is increased, the convergence state of laser beam deteriorates due to coma aberration with respect to the inclination of the optical disk Since coma aberration is proportion to the third power of the numerical aperture of the objective lens and to the thickness of the disk substrate, the thickness of the disk substrate of DVDs is designed to be about 0.6 mm, which is half that of CDs.
When the thickness of the substrate deviates from the designed value, spherical aberration occurs at a convergence position of light passing through the inward portion of the objective lens and a convergence position of light passing through the outward portion. Therefore, when CD is read by the objective lens with a numerical aperture of 0.6 optimized to the thickness of the DVD substrate, it is necessary to correct the spherical aberration by limiting the outward luminous flux incident on the lens or by slightly diverging the incident angle at the lens.
Thus, one objective lens can be used in common for the DVD and the CD with the necessary correction of spherical aberration, but two laser beam sources each having a different wavelength from other have to be provided for compatibility with a write-once-read-many CD. This is because a reflective recording layer of the write-once-read-many CD is formed of an organic dye material and thus has a reflection coefficient as low as 6% for laser beam having a wavelength of 635 nm to 650 nm, that is a wavelength appropriate to the DVD.
Thus, since the current DVD optical head apparatus is equipped with two laser beam sources respectively with a wavelength of 635 nm to 650 nm for the DVD and a wavelength of 780 nm for the CD, and since laser beams from the two light sources are to be guided to two respective objective lenses, parts such as a prism, aperture control means, and the like am required for respective laser beams, thereby prohibiting downsizing and cost reduction of the apparatus.
In order to solve the problems described above, various optical pickup apparatuses shown in FIGS. 10 to 13 have been proposed. Conventional optical pickup apparatuses will be outlined below.
FIG. 10 is a block diagram of a first conventional example. There are provided laser beam sources 91 and 12 to emit laser beams with a wavelength of 650 nm for the DVD and a wavelength of 780 nm for the CD, respectively, a wavelength selection prism 92 to guide any one of the laser beams along a same optical path, and a half mirror 11 to reflect and guide the laser beam to a collimating lens 13 and also to pass and guide a reflected laser beam from a disk 18 to a photodetector 90. There is further provided a reflection mirror 15 to direct the laser beam having passed through the collimating lens 13 to an objective lens 16 or 17 to converge the laser beam onto the disk 18 (either DVD 18a or CD 18b) placed on a drive mechanism (not shown) and rotated thereby.
The objective lens 16 has a high numerical aperture high NA) for DVDs, and the objective lens 17 has a low numerical aperture pow NA) for CDs. The drive mechanism (not shown) is adapted to select from and switch over between the objective lens 16 and 17 for DVDs and CDs. The laser beam reflected at the disk 18 and returning therefrom passes through the half mirror 11 and is received by the photo-detector 90 that converts it into an electrical signal.
FIGS. 9A and 9B represent schematically the wavelength selection prism 92. The wavelength selection prism 92 is provided with an optical path control film 80 having wavelength transmission characteristic as shown in FIG. 9C. The optical path control film 80 is predisposed to block light having a wavelength of 700 nm and less, and to transmit light having a wavelength of 750 nm and more. Therefore, while light 81 with a wavelength of 780 nm incident on the optical path control film 80 is not blocked by the optical path control film 80 and travels straight through as shown in FIG. 9A, light 82 with a wavelength of 650 nm incident on the optical path control film 80 orthogonally to the light 81 is blocked by the optical path control film 80 and reflected by 90 degrees to be directed along the same optical path as the light 81 as shown in FIG. 9B.
The optical pickup apparatus of the first conventional example shown in FIG. 10 operates as follows. The laser beam source (laser diode (wavelength: 650 nm)) 91 for DVDs and the laser beam source (laser diode (wavelength: 780 nm)) 12 for CDs are disposed orthogonal to each other so that respective laser beams therefrom are guided into the same optical path by the wavelength selection prism 92. The laser beam has its optical axes reflected by 90 degrees by the half mirror 11, is converted into a parallel pencil by the collimating lens 13, and reflected and directed to the objective lens 16 or 17 by the reflecting mirror 15.
As described above, the objective lens 16 with a high NA for DVD and the objective lens 17 with a low NA for CD are switched over by means of the drive mechanism (not shown). When reading a DVD, the laser diode 91 for DVDs oscillates, and the objective lens 16 with a high NA for DVDs is placed on the optical path to converge the laser beam onto the disk (DVD) 18a. When reading a CD, the laser diode 12 for CDs oscillates, and the objective lens 17 with a low NA for CDs is placed on the optical path to converge the laser beam onto the disk (CD) 18b. The above-described switching mechanism is incorporated into an axial-displacement-type actuator apparatus (not shown). The laser beam reflected at either disk starts traveling in the backward direction along the incoming path, passes through the half mirror 11, and is directed to the photo-detector 90 to be converted into an electrical signal.
FIG. 11 is a block diagram of a second conventional example, in which laser diodes for DVDs and CDs and a photo-detector are packaged into an enclosure thereby constituting an integrated laser unit, thus making it possible to reduce the number of components in comparison with the first conventional example. Specifically, an integrated laser unit 201 for CDs includes integrally a laser beam source with a wavelength of 780 nm appropriate for CDs and a photodetector for CDs, and an integrated laser unit 202 for DVDs includes integrally a laser beam source with a wavelength of 650 nm appropriate for DVDs and a photo-detector for DVDs. Laser beams from the integrated laser unit 201 for CDs and the integrated laser unit 202 for DVDs are each made incident on a wavelength selection prism 92. This optical pickup apparatus operates in the same way as the first conventional example in FIG. 10, and the description thereof is omitted.
FIG. 12 is a block diagram showing a third conventional example, which differs from the first conventional example in that there are provided: an aperture control filter 103 disposed immediately before an objective lens 16; a wavelength selection prism 92 for separating a laser beam reflected at the disk according to the wavelength of the laser beam; and two photo-detectors 101 for CDs and 102 for DVDs each for receiving a laser beam from the wavelength selection prism 92.
FIG. 7 represents schematically the aperture control filter 103. An anti-reflection film AR is formed entirely on one surface 30A of a sheet glass 60 and partly only at a central portion on the other surface 30B, and a wavelength selection film 61 is formed at other portion than the central portion on the other surface 30B, where an anti-reflection film AR is not formed.
FIG. 8 shows the relationship between the wavelength and the transmittance in the wavelength selection film 61 used in the aperture control filter 103. As clear in FIG. 8, the transmittance decreases for wavelengths from 725 nm upward. This means that the laser beam with a wavelength of 780 nm appropriate for CDs is reflected.
The optical pickup apparatus of the third conventional example shown in FIG. 12 operates as follows. A laser diode (wavelength: 650 nm) 91 for DVDs and a laser diode (wavelength: 780 nm) 12 for CDs as light sources are disposed orthogonal to each other so that respective laser beams emitted from the laser diodes 91 and 12 are introduced into the same optical path by the wavelength selection prism 92. The laser beam has its optical axes direction changed by 90° by the half or 11, is formed into a parallel luminous flux by a collimating lens 13, reflected by a reflecting mirror 15 toward a disk 18 (18a, 18b), passes through the aperture control filter 103, and is made incident on the objective lens 16 having a high numerical aperture for DVDs. The objective lens 16 functions in the same way as described for FIG. 11, and the description thereof is omitted.
As described above, the aperture control filter 103 has different optical film characteristics between at its central circular portion and at the other portion therearound. While the other portion transmits light with a wavelength of 650 nm and reflects light with a wavelength of 780 nm, the central circular portion transmits both. When reading DVDs, the luminous flux is not affected by the aperture control filter 103 and is entirely made incident on the objective lens 16 to be converged onto the disk 18a. When reading CDs, the luminous flux is affected by the aperture control filter 103 to pass therethrough part all at its central circular portion, and is made incident on the objective lens 16, whereby the effective NA is decreased, and light is converged onto the disk 18b with a low aberration.
However, the spherical aberration is still present after only limiting the aperture to decrease the effective NA. The laser diode 12 for CDs must be positioned closer to the collimating lens 13 so as to cancel the spherical aberration, so that a light beam is made incident on the objective lens 16 with slight divergence. Therefore, the converging position of reflected light from the disk 18 varies according to the distance therebetween, so the distance to the photo-detector 101 for CDs cannot be identical with the distance to the photodetector 102 for DVDs, thus requiring two photo-detectors. The two photo-detectors receive respective light beams having a different wavelength from each other due to the wavelength selection prism 92.
FIG. 13 is a block diagram of a fourth conventional example, in which an integrated laser unit 201 for CDs and an integrated laser unit 202 for DVDs each having a laser diode and a photo-detector packaged into an enclosure are used, and the number of components can be reduced in comparison with the third conventional example. The integrated laser unit 201 for CDs and the integrated laser unit 202 for DVDs are configured in the same way as those in FIG. 11, and the description thereof is omitted. And, they operate in a similar way to those of the third conventional example shown in FIG. 12, and the description thereof is omitted.
The apparatuses of the conventional examples have the following problems.
In the first conventional example, two laser diodes with different wavelength are required for ensuring the compatibility among DVDs, CDs, CD-R/RWs(CD Recordable/Re-Writable), and a means to introduce two laser beams into the same optical path is also required.
In the second conventional example, in addition to the problem associated with the first conventional example, expensive integrated laser units are required, resulting in difficulty with cost reduction.
In the third conventional example, two laser diodes with different wavelengths are required, and the cost of the aperture control filter increases for compensating the phase difference generated between two kinds of films showing transmission characteristics different from each other, resulting in difficulty with cost reduction.
In the fourth conventional example, in addition to the problem associated with the third conventional example, expensive integrated laser units are required, also resulting in difficulty with cost reduction.